The spread of mobile and portable electronic devices, such as cell phones and notebook computers, in recent years has been accompanied by a strong desire for the development of nonaqueous electrolyte secondary batteries that are small, light, and have a high energy density. Lithium ion secondary batteries are such a secondary battery. Materials capable of the extraction insertion of lithium are used for the positive electrode and negative electrode materials of lithium ion secondary batteries.
Research and development is also currently being actively carried out into the positive electrode materials in such lithium ion secondary batteries. Within this sphere, lithium ion secondary batteries that use a lithium metal complex oxide—and particularly the relatively easy-to-synthesize lithium cobalt complex oxide (LiCoO2)—for the positive electrode material provide high voltages in the 4 V class and for this reason their practical application as high energy density batteries is moving forward. A great deal of research has been carried out to date in order to obtain excellent initial capacity characteristics and excellent cycle characteristics from lithium ion secondary batteries that use this lithium cobalt complex oxide, and various outcomes have already been obtained.
However, expensive and production-constrained cobalt compounds are used as starting materials for lithium cobalt complex oxide, which causes higher battery costs. As a consequence, there is desire for the use of something other than lithium cobalt complex oxide as a positive electrode active material.
In addition, aside from use as small-scale secondary batteries for mobile and portable electronic devices, expectations have also been increasing with regard to the application of lithium ion secondary batteries as large-scale secondary batteries, for example, for power storage and electric vehicles. As a consequence, ripple effects into a broad range of fields can be expected if the active material costs can be reduced to thereby make possible the production of less expensive lithium ion secondary batteries.
Lithium manganese complex oxide (LiMn2O4) and lithium nickel complex oxide (LiNiO2) are examples of materials that have been newly introduced as positive electrode active materials for lithium ion secondary batteries; these use, respectively, manganese and nickel, which are less expensive than cobalt.
Due to the low cost of its starting materials and its excellent thermal stability and particularly its excellent safety with regard to, e.g., ignition, lithium manganese complex oxide can be regarded as a strong substitute material for lithium cobalt complex oxide. However, since its theoretical capacity is only about one-half that of lithium cobalt complex oxide, one problem associated with lithium manganese complex oxide is the difficulty in responding to the ever increasing requirements for higher capacities for lithium ion secondary batteries. Another problem is that, at 45° C. and above, substantial self-discharge occurs and the charge/discharge life is also reduced.
Lithium nickel complex oxide, on the other hand, has about the same theoretical capacity as lithium cobalt complex oxide and exhibits a battery voltage somewhat lower than that of lithium cobalt complex oxide. As a consequence, the problem of oxidative decomposition of the electrolyte solution is suppressed and a higher capacity can be expected and active development has been underway as a result. However, one problem is that a lithium ion secondary battery fabricated using a lithium nickel complex oxide formed using only nickel by itself—without substituting another element for the nickel—as its positive electrode active material has cycle characteristics that are inferior to those for lithium cobalt complex oxide. Another problem has been that the battery performance is relatively easily impaired by use or storage in a high-temperature environment.
In order to solve these problems, for example, Patent Documents 1 to 3 introduce, with the goal of maintaining a good battery performance during storage or use in a high-temperature environment, lithium-containing complex oxides in which a portion of the nickel in lithium nickel complex oxide has been substituted by an element such as boron, cobalt, or aluminum. For example, certain effects, such as a suppression of the decomposition reactions of the positive electrode active material and an improvement in the thermal stability, have been confirmed when aluminum is selected as the actual substituting metal and a large amount of the nickel is substituted by aluminum. An improvement in the cycle characteristics has been confirmed for the substitution of a portion of the nickel with cobalt.
These is no doubt that these substituting elements are useful for solving some of the problems associated with lithium nickel composite oxide; however, in order to bring out the intrinsic properties of lithium nickel complex oxide, it is most important that the crystal structure of lithium nickel complex oxide be finely controlled, and proposals have been made to improve the battery characteristics by tuning the crystal structure.
For example, Patent Document 4 discloses a positive electrode active material in which the nickel occupancy rate at the 3a site in the crystal structure is 1.5 to 2.9%. Patent Document 5 discloses a positive electrode active material that can—by having the lithium occupancy rate at the 3a site be at least 98.5% and the metal occupancy rate at the 3b site be 95 to 98% —simultaneously achieve an increase in the capacity and an increase in the output.
However, a problem with the controlled-crystal structure lithium nickel complex oxides as described above is their large irreversible capacity, which is the difference between the initial charging capacity and the initial discharge capacity. The appearance of a large irreversible capacity, i.e., a low initial charge/discharge efficiency, has required that the capacity of the negative electrode be raised in order to absorb the irreversible capacity fraction and has been an impediment to raising the battery capacity since the capacity per unit volume for the battery as a whole ends up declining.
For example, Patent Document 6 discloses a positive electrode active material that is characterized by a lithium ion occupancy rate at the 3a site according to the results of Rietveld analysis in x-ray diffraction of at least 97%. It is suggested that the lithium occupancy rate at the 3a site influences the irreversible capacity and that it is possible to raise the initial charge/discharge capacity and reduce the irreversible capacity by bringing about an increase in this lithium occupancy rate. However, an initial charge/discharge efficiency greater than 90.0% has not been obtained and the irreversible capacity in the initial charge/discharge of lithium nickel complex oxide has still remained large, and obtaining an initial charge/discharge efficiency of greater than 90% has been problematic.
Moreover, vehicle-mounted batteries—as in hybrid vehicles (HEV), which combine two types of drive power sources (gasoline engine and electric motor), and electric vehicles (EV)—are required not only to have a high capacity, but to also exhibit high output characteristics, i.e., an excellent rate characteristic.
For example, Patent Document 7 discloses a nonaqueous electrolyte secondary battery that has excellent output characteristics; this nonaqueous electrolyte secondary battery has a first positive electrode active material that has a high Ni content and a large average particle diameter, a second positive electrode active material that has a low Ni content and a small average particle diameter, and a first electroconductive auxiliary agent and a second electroconductive auxiliary agent having different average particle diameters. However, this does not reach to an improvement in the rate characteristic of lithium nickel complex oxide itself.